Progressive wave techniques have been used to determine the dynamic material constants of rubber compounds. These techniques commonly use piezoelectric transducers which contact lightly on the surface of a test strip. The strip is excited into oscillation by an electromechanical shaker. One technique is to use two piezoelectric transducers in contact with the test strip. The transducers measure the amplitude and phase differences for a given length of the test strip. In a second technique the strip is excited into oscillation by an electromechanical shaker and a single piezoelectric transducer is moved along the surface of the test strip so that a continuous record is made of the amplitude level and relative phase as a function of distance along the test strip at one frequency. From the record obtained, the attenuation or loss factor is determined and the wavelength is determined between points of equal relative phase along the strip. Since the frequency is known, the propagation velocity can be determined.
The prior art progressive wave techniques are difficult to use since the data is taken by hand at each discrete frequency making the measurements a very time-consuming process. A disadvantage of the first technique is that the responses of the two piezoelectric transducers are not equal and consequently produce large errors in the amplitude and phase measurements. A serious disadvantage of the second technique is that the piezoelectric transducer is not only sensitive to the response of the excited strip but is also sensitive to variations in the surface roughness of the strip which introduces non-systematic errors into the measurements.